Add flake module

[roberth]

The flakeModule attribute allows the project to be integrated easily into flakes based on flake-parts, which is a minimal set of module system options that unifies many flake integrations.

Besides brevity and easy composition, this solves a problem for users who have trouble integrating pkgs, as it's not trivial to go from the template to a well integrated call where pkgs has a customizable value.

I'll publish the option documentation on flake.parts.

(The broken site seems to be a pre-existing problem)

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